exploring magnetism - amazon web...
TRANSCRIPT
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EXPLORING MAGNETISM
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What is a Magnet?
What Materials are Magnetic?
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What Materials are Magnetic?
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Magnets and magnetic materials
Magnets attract objects made of magnetic materials
Magnetic materials include the elements iron, nickel, cobalt, alloys containing some of these such as steel and some of their compounds.
stainless
steel
cobalt nickel iron
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Hard and soft magnetic materials
Permanent magnets are made of magnetically HARD materials such as steel. These materials retain their magnetisation once magnetised.
Magnetically SOFT materials, such as iron, lose their magnetisation easily. They suitable for temporary magnets such as electromagnets.
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What Do Magnets Do?
Attract or repel other magnets (exert a
force)
Attract other magnetic metals
Have at least 2 distinct ends (poles) each
Like poles repel, opposite poles attract
Notes:
Do not need to touch to exert force (can exert force
through empty space)
Can turn other magnetic metal objects into temporary
magnets
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Magnetic poles
Magnetic poles are the parts of a magnet that exert the greatest force.
Magnetic poles occur in pairs usually called north (N) and south (S)
Iron filing are attracted
mostly to the poles of
a magnet
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Why poles are called north and south A magnet suspended so that it can rotate freely horizontally will eventually settle down with one pole facing north and the other south.
This is pole is therefore called the ‘north seeking pole’, usually shortened to just ‘north pole’.
The magnet has been orientated by the Earth’s magnetic field.
A compass is an application of this effect.
north
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The law of magnets
Like poles repel unlike poles attract
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Magnetism in a Magnet
When elements like Fe form solids, they form crystalline
structures with little domains of many, many atoms all
aligned. Each domain is like a mini-bar magnet. When all
the domains are allowed to align, the whole object
takes on a large-scale magnetic field.
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Moving molten
iron in Earth’s
outer core
causes most of
Earth’s magnetic
field.
Magnetic field
poles are NOT
aligned with
geographic
poles. They also
wander and flip
(500,000 yrs
between flips,
10,000 yrs to
complete flip).
Earth: A Huge Bar
Magnet
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What is a Force?
Force is a push or pull that causes a
change in motion.
A Force Field is the pattern in space of
how that force is felt by other objects.
Fundamental Forces arise from
fundamental properties of matter:
Gravity arise
from Mass: mass
attracts mass
Electric force
arises from
Charge: two
kinds of charge
(positive and
negative), like
charges repel
and opposite
charges attract
Gravitational
field of a point
of mass Electric field of 2 opposite
charges
Electric field of 2 like charges
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Magnetic Field
Magnetic field of a Bar
Magnet: 2 poles, called
North and South
Dipole field
Field has direction: lines
point away from N and
toward S
Definition of a Pole: Where lines meet
(converge)
e.g.: lines of longitude on a globe meet
at poles
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16th C, William Gilbert
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1820, Hans Oersted
• …connected a
battery to let
electric current
flow, and noticed
a compass
needle twitch and
move.
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Oersted’s Experiment
Hans Christian Oersted discovered in 1820 that an electric current near a compass causes the compass needle to be deflected. Oersted's experiment showed that every electric current has a magnetic field surrounding it.
Hans Christian Oersted
(1777-1851)
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Oersted’s Experiment
According to the right-hand rule, the electron current
in a wire and the magnetic field it generates are
perpendicular to each other.
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Oersted’s Experiment
All magnetic fields originate from moving electric
charges. A magnetic field appears only when relative
motion is present between an electric charge and an
observer. Electric and magnetic fields are different
aspects of a single electromagnetic field.
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Electric Current (Electricity) is
moving charge.
In an electric circuit, batteries
provide an electric field to push
charges through a wire, which
provides a path for them to flow.
Unless there is a complete
circuit the charges cannot flow.
Compasses around a circuit with
flowing electric current detect a
magnetic field.
Magnets can push or pull on a
wire with current flowing through
it.
Electricity and Magnetism
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Electricity and Magnetism
The
shape of
the
magnetic
field
around a
long,
straight
wire
carrying
electric
current
Looping
the wire
turns the
magnetic
field into
a toroidal
(donut)
shape.
Many loops
in a solenoid
shape
creates a
dipole-
shaped
magnetic
field, similar
to that of a
bar magnet
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Electricity and Magnetism
A dynamic electric field creates
a magnetic field.
A dynamic magnetic field creates
an electric field.
The basic principle behind
electric generators is creating
relative motion between a
magnet and a wire to create an
electric field that will push
current through a circuit.
Converts kinetic energy into
electric energy.
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The genius of Faraday
• In London (in a lab I visited in 1997) Michael Faraday (1791-1867), an unschooled bookbinder’s assistant, experimented with magnets and currents.
• In 1831 he observed that a moving magnet could induce a current in a circuit.
– This is the inverse of Oersted’s observation.
– Somehow electricity and magnetism were intimately related!!!
• This became Faraday’s law of induction and ultimately one of Maxwell’s equations
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Other events of 1831 • Faraday also observed that a changing current
could, through its magnetic effects, induce a current to flow in another circuit.
• If you spin a magnet inside a circuit it will generate current – the electric generator.
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The real genius of Faraday
• Since he had no mathematical training but thought geometrically, he
• invented the concept of fields of force.
– A geometric means of conceiving of what his experiments were showing him.
• This concept, this interpretation of what he saw is what set him apart from his predecessors.
• It enables modern science!!!!
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James Clerk Maxwell
• James Clerk Maxwell had the mathematical skills that Faraday lacked and used them to become the greatest theoretician of the 19th century.
• He graduated Edinburgh University at age 15 and became a full professor at Aberdeen University at age 17.
• In the 40 years (1839-1879) of his life he established the foundations of electricity and magnetism as
• electromagnetics, established the kinetic theory of gasses, explained the rings of Saturn and experimented with color vision.
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Maxwell’s symmetry and unification
• Two rules governed electricity and two other rules governed magnetism.
• Maxwell noticed that in these laws the electric field and the magnetic field appeared nearly symmetrically in the equations.
• For example, in Faraday’s Law a time varying magnetic field gave rise to an electric field.
• In Ampere’s law, as Maxwell modified it, a time varying electric field gave rise to a magnetic field.
• When made symmetric in electric and magnetic fields the set of four equations described them both, they described the subject we now call electromagnetism.
• Electricity and magnetism had been unified into electromagnetism!
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Magnetic fields
A magnetic field is a volume of space where magnetic force is exerted.
All magnets are surrounded by magnetic fields.
The shape of a magnetic field can be shown by iron filings or plotting compasses.
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Magnetic field around a bar magnet magnetic field line Arrows on the field
lines show the
direction of the force
on a free to move
north pole
The stronger the
magnetic field the
denser the magnetic
field lines.
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Magnetic fields between two bar magnets
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Producing a uniform magnetic field
A uniform magnetic field exerts a constant force over a region.
Such a field will consist of parallel equally spaced magnetic field lines.
This type of field can almost be found between a north and south magnetic pole.
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Induced magnetism Magnetism can be induced in a magnetic material if it is placed within a magnetic field.
If the material is magnetically hard it will retain its magnetism once removed from the field.
Certain rocks in the Earth’s crust such as lodestone have been magnetised in this way by the Earth’s magnetic field.
iron bar
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Magnets and Magnetic Fields
Magnetic fields can be visualized using
magnetic field lines, which are always closed
loops.
Notice that the
magnetic field
goes from
North to South.
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Magnets and Magnetic Fields
The Earth’s magnetic field is similar to that of a
bar magnet.
Note that the Earth’s
“North Pole” is really
a south magnetic
pole, as the north
ends of magnets are
attracted to it.
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Magnets and Magnetic Fields
A uniform magnetic field is constant in
magnitude and direction.
The field between
these two wide poles
is nearly uniform.
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Electric Currents Produce Magnetic Fields
Experiment shows that an electric current
produces a magnetic field. (Right Hand Rule)
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Magnetic field lines due to a circular loop of wire
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Electric Currents Produce Magnetic Fields
The direction of the
field is given by a
right-hand rule.
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Force on an Electric Current in a Magnetic
Field; Definition of B
A magnet exerts a force on a current-
carrying wire. The direction of the force is
given by a right-hand rule.
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Magnetic Force on a Current
A magnetic field exerts a
sideways push on an
electric current with the
maximum push occurring
when the current is
perpendicular to the
magnetic field. Currents
exert magnetic forces on
each other. The forces are
attractive when parallel
currents are in the same
direction and are repulsive
when the parallel currents
are in opposite directions.
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The experimental Japanese Maglev train uses magnetic forces for both support and
propulsion.
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Magnetic field patterns around wires
1. Straight wire
The magnetic field consists
of concentric circles centred
on the wire.
The magnetic field is
strongest near the wire.
This is shown by the field
lines being closest together
near to the wire.
The strength of the field
increases if the eclectic
current is increased.
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The right-hand grip rule (for fields)
Grip the wire with the
RIGHT hand.
The thumb is placed in
the direction of the
electric current.
The fingers show the
direction of the
circular magnetic field.
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Complete the diagrams below:
Electric current
into the page Electric current
out of the page
Add
field
arrow
s
Add
field
arrow
s
Add
current
directio
n
Add
current
directio
n
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2. Flat circular coil
Plan view
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3. Solenoid
A solenoid is a coil of wire
carrying an electric current.
The magnetic field is similar in
shape to that around a bar
magnet.
The strength of the field
increases with:
1. the electric current
2. the number of turns in the coil
N S
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The right-hand grip rule (for poles)
Grip the coil with the
RIGHT hand.
The fingers are placed
in the direction that
the eclectic current
flows around the coil.
The thumb points
towards the north pole
end of the coil.
N S
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Complete the diagrams below:
1. Locate
north
4. Add
coils
3. Add current
direction
2. Locate
south
N
S
N
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Current-carrying wire in a magnetic field. The force on
the wire is directed into the page.
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Force on an Electric Current in a Magnetic
Field; Definition of B
The force on the wire depends on the
current, the length of the wire, the magnetic
field, and its orientation.
(on formula sheet)
If the direction of I is perpendicular to B,
then θ = 90˚ and sin θ = 1. If I is parallel to
B, then θ = 0˚ and sin θ = 0. This equation
defines the magnetic field B.
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Force on an Electric Current in a Magnetic
Field; Definition of B
Unit of B: the tesla, T.
1 T = 1 N/A·m.
Another unit sometimes used: the gauss (G).
1 G = 10-4 T.
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Measuring a magnetic field B
What is the magnetic
field if the force is
0.0348 N and the
current is 0.245 A?
What about the
magnetic forces on
the two vertical
sections of the wire
that are in the
magnetic field?
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Force on Electric Charge Moving in a
Magnetic Field
The force on a moving charge is related to
the force on a current: (on formula sheet)
Once again, the direction
is given by a right-hand
rule. The rule is for
positive particles. Notice
the difference between
positive and negative
particles.
The force is greatest when the particle moves perpendicular to B
(θ = 90°)
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20.4 Force on Electric Charge Moving in a
Magnetic Field
If a charged particle
(in this case an
electron) is moving
perpendicular to a
uniform magnetic field,
its path will be a circle.
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What is the path of a charged particle if its velocity is not
perpendicular to the magnetic field?
The parallel component of v experiences no force, so it
remains constant. The perpendicular component of v results
in circular motion. Together this produces a spiral motion.
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Charged ions approach the Earth from the sun (solar wind) and
are drawn toward the poles. This causes the Northern lights.
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Force on Electric Charge Moving in a
Magnetic Field
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Magnetic Field Due to a Long Straight
Wire
The field is inversely proportional to the
distance from the wire:
(on formula sheet)
The constant μ0 is called the permeability of
free space (or Vacuum permeability), and has
the value:
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Force between Two Parallel Wires
The magnetic field produced
at the position of wire 2 due to
the current in wire 1 is:
The force this field exerts on
a length l2 of wire 2 is:
(20-6)
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Force between Two Parallel Wires
Parallel currents attract; antiparallel currents
repel.
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Solenoids and Electromagnets
If a piece of iron is inserted in the solenoid, the
magnetic field greatly increases. Such
electromagnets have many practical
applications.
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Electromagnetic Induction
The effect of producing an
induced current is known as
electromagnetic induction.
The direction of the induced
current can be reversed by
reversing the motion of the wire
or reversing the field direction.
The strength of the current
depends on the strength of the
magnetic field and the speed of
the wire's motion.
Induction Movie
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Electromagnets
An electromagnet consists of an iron core placed
inside a wire coil. The magnetic field strength of a
wire coil carrying an electric current increases in
direct proportion to the number of turns of the coil.
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Electric Motors
An electric motor uses
the sideways push of
a magnetic field to
turn a current-carrying
wire loop. Electric
motors use a
commutator to
change the direction
of the current in the
loop. Alternating
current electric motors
do not use
commutators.
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Applications: Motors & Loudspeakers
An electric motor
takes advantage of
the torque on a
current loop, to
change electrical
energy to
mechanical energy.
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Applications: Motors & Loudspeakers
Loudspeakers use the
principle that a magnet
exerts a force on a
current-carrying wire to
convert electrical
signals into mechanical
vibrations, producing
sound.
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Alternating and Direct Current
Alternating current (ac) is current that flows in a back-and-
forth manner; household current changes direction 120 times
each second (60 Hz). Direct current (dc) flows in one
direction.
The ac generator (or alternator) produces an ac current and can be modified to produce dc current by
1. Use of a commutator.
2. Use of a rectifier which permits current to pass through it in only one direction.
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Transformers
A transformer is a device composed of two unconnected
coils, usually wrapped around a soft iron core, that can
increase or decrease the voltage of ac current.
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Transformers
A transformer is used to step the voltage
down and the current up (P=IV) so that
we can use it. Low power is desired for
the transport of electricity long distances
to avoid loss of energy to heat loss.
Transformers must use AC current.
A moving coil activated by voice vibrations is used as a microphone. The coil induces a current in the magnet that can be amplified or recorded.
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Transformers
A taperecorder records signals from a microphone on
magnetic tape which then can be run across a magnet and
played back.
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Electromagnets
An electromagnet consists of a
current carrying coil wrapped
around an iron coil.
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Uses of electromagnets
1. Scrap yard crane
The iron core of the electromagnet
is a SOFT magnetic material.
When current flows the iron
becomes strongly magnetised and
so picks up the scrap iron and
steel.
When the current is turned off the
iron loses its magnetisation and so
releases the scrap.
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The electric bell
When the push switch is closed current flows around the circuit turning on the electromagnet.
The soft iron armature is pulled towards the electromagnet and the hammer hits the gong.
This causes the contact switch to open cutting off the electric current.
The spring now pulls the armature back again closing the contact switch.
Current now flows again and the hammer hits the gong again.
gon
g
ham
mer
electrom
agnet
push
swit
ch
spri
ng
conta
ct
switc
h
soft
iron
armatu
re
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Label the diagram of the electric bell below:
1
2
3
4
5
6
Contact
switch
7
8
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The relay switch A relay switch is a way of using a low voltage circuit to switch remotely a high voltage (and possibly dangerous) circuit.
When switch A is closed, the small current provided by the cell causes the electromagnet to become magnetised..
The iron armature is then attracted to the electromagnet causing the springy contact switch B to close in the high voltage circuit.
switch A
springy
contact
switch B
iron
armature
hinge
electromagnet
to high
voltage
circuit
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A
B
4. Circuit breaker Current normally flows between terminals A and B through the contact and the electromagnet.
When the current in a circuit increases, the strength of the electromagnet will also increase. This will pull the soft iron armature towards the electromagnet.
As a result, spring 1 pulls apart the contact and disconnecting the circuit immediately, and stopping current flow.
The reset button can be pushed to bring the contact back to its original position to reconnect the circuit
1
2
Domestic
circuit
breakers
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Electromagnetic Radiation
Electromagnetic waves travel through empty space at a
speed of 300,000 km/s (186,000 miles/sec).
EM waves with wavelengths 400-700 nm are seen by the
human eye as Visible Light.